Aquatic and semiaquatic mammals

I've mentioned elsewhere that AAT/H proponents are typically extremely coy about what aquatic animals we're supposed to resemble; they're also somewhat idiosyncratic about what they say constitutes an "aquatic" or "semiaquatic" mammal.
Marc Verhaegen has often claimed that non-aquatic mammals such as rhinoceros and mountain beavers are aquatic, which is nonsense.
Others claim pigs are; again nonsense.
Many times they don't really like to say at all, although they don't spend much time looking at actual aquatic and semiaquatic mammals, preferring vagueness, merely claiming that humans share hair and fat characteristics with "aquatics" or that such characteristics are found in "an aquatic environment".
But if you take the step of looking at what animals they have to be talking about they get very defensive, because if you do you end up thinking "how much like a seal am I really?".

The general AAT/H vagueness about degree of water use has followed this general arc:

a fairly vague statement by Hardy (less aquatic than otters for maybe 10 million years);

virtually nothing beyond "as aquatic as an otter" or "less aquatic than an otter" for the next forty years (except for Hardy in 1978 making his earlier statement far more explicit (now 5-6 hours in water every day for 20 or so million years); okay, it's completely impossible -- longer than hominids of any kind have existed -- but at least he said what he was thinking);

Algis Kuliukas' formulation of the "more aquatic than apes" phrase (sometimes specifically chimps); his definition is "...that water has acted as an agent of selection in the evolution of humans more than it has in the evolution of our ape cousins..." (the full definition is on the page about him on this site).
This is so vague that's it's meaningless.

I mean really, water has acted as an "agent of selection" in the evolution of wolves and black bears more than it has in the evolution of our ape cousins, and wolves and black bears aren't semiaquatic nor were their ancestors.

That can be contrasted with typical scales of aquaticness used by actual zoologists.
For instance, when I was recently making a list of semiaquatic and aquatic mammals one source I used had this as a note for a table of aquatic and semiaquatic mammals:

From Table 1.4 by Henry Pihlström, "Introduction: On Becoming Aquatic" in Sensory Evolution on the Threshold: Adaptations in Secondarily Aquatic Vertebrates edited by J. G. M. Thewissen and Sirpa Nummela
Publisher: University of California Press

Or consider the explanation set forth in the "Methods" section of a 2001 paper by Bininda-Emonds et al.

("Flippers versus feet: comparative trends in aquatic and non-aquatic carnivores", Journal of Animal Ecology, 2001, 70:386-400): "We consider aquatic carnivores to be those species in which the aquatic habitat inevitably plays a key role in the life-cycle of an individual..."

Because the AAT/H claims that aquatic and semiaquatic living creates selection for hairlessness and fatness, I thought it would be useful to have a list of all the aquatic and semiaquatic mammals, and I'm also going to look at how many times aquaticism has evolved in aquatic and semiaquatic mammals and how many are hairless and/or fatty.
The list totals 203 species; for my purposes it's broken down as follows:

112 hairy; 75 not including seals and walrus

91 hairless (84 cetaceans, 5 sirenia, 2 hippos)

125 "fatties" (84 cetaceans, 5 sirenia, 36 seals and walrus)

35 of the hairy ones tropical (or in the case of 2 or 3 species ranging into the tropics or subtropical areas)

4 of the species listed are now (relatively recently) extinct

The reason I've broken it down that way is the claims I've seen used over the years by AAT/H proponents and what they've said when their claims get critiqued. Two of the basic claims are that natural selection (and pointedly not sexual selection or genetic drift or the demands of thermoregulation) and a watery environment is behind humans being "hairless" and relatively fat. (They don't accurately describe human hair and fat characteristics but that's another mark against their ideas.) AAT/H proponents generally do this without saying what aquatic and/or semiaquatic mammals they're talking about; typically they just claim water use does the deed. Typically they also refuse to say how much water use this takes, sometimes going to comical lengths to not say. Looking at this list gives you an idea of just what amount of water use it takes to, say, lose body hair.

"Fatties" refers to generally a good deal more body fat than is "average" among wild mammals such as primates, bovidae, most carnivores, and such; the average more or less seems to be around 5-10%. Naturally this varies a lot seasonally and with the health of the animal and how its food supply is, how much predation pressure they face, etc. so it is approximate, but we're looking for year-round "fatties" here. For this purpose we're also ignoring the many fatty non-semiaquatic mammals -- such as badgers, bears, pigs, camels, and hedgehogs -- just as AAT/H proponents virtually always do.

When you point out that semiaquatic mammals are often hairy, AAT/H proponents sometimes claim that of course our early ancestors were tropical and only cold weather semiaquatic mammals are hairy.
This is not true, so to point this out in the full list I've marked the tropical semiaquatic mammals, or those that range into the tropics or subtropics. Contrary to the common AAT/H claim, except for cetaceans, sirenia, and hippos they're all hairy.

How many times has aquaticism or semiaquaticism arisen among extant mammals?

I've got the full list of all aquatic and semiaquatic mammal species at the bottom of this page, and it's pretty long, but first there's the question of how many times hairlessness evolved among aquatic and semiaquatic mammals. Since AAT/H proponents typically do their best to ignore hairy aquatic and semiaquatic mammals entirely they never really get around to this question, but it's interesting. What makes it interesting is that there are two reasons the AAT/H claim that water use begets hairlessness falls down.

The first is the degree of water use; even if you assume that water use rather than body size and thermoregulation is behind all these species' hairlessness you have only species which have been aquatic for tens of millions of years and which are highly aquatic, except for the hippos. With hippos it's generally acknowledged that the well known physics of body volume vs. surface is behind their hairlessness.
For those who aren't aware of how this works, it's the same principle that a radiator works on; all those fins on a radiator dissipate heat because there's more surface area to volume. Large animals with stocky bodies tend to retain heat, and they're helped by losing that hair. Examples are hippos, rhinos, and elephants. Further evidence that this is why they're relatively hairless is the ice age far north versions of the elephant and the rhino, the wooly mammoth and the wooly rhinoceros. Of course cetaceans and sirenia are also almost all very large and have relatively small limbs in the form of flippers; they'd have a big problem with getting rid of heat if they weren't in the water virtually all the time, and in fact when these animals are transported or beached a major problem for them is overheating. Whether their ancestors lost body hair because of swimming (which seems especially unlikely in the case of slow-moving sirenia) or just body size is not known.

The other question is how commonly hairlessness is connected with aquaticism at all. Even if you just add up how many aquatic and semiaquatic mammal species are hairless, the total is less than half. But it's just not possible that each and every present-day aquatic and semiaquatic mammal independently evolved aquaticness. We know that many of them have aquatic ancestors and have evolved from a long line of aquatic ancestors, and this is especially true for those which are hairless; it's incredibly unlikely that their aquatic ancestors were hairy for tens of millions of aquatic years and then after diverging they all became hairless. And of course even if that incredibly unlikely scenario were the case it would hurt rather than help the AAT/H argument. So the question is how many times has aquaticness evolved, and in the context of the AAT/H claims, how many times for hairless mammals and how many times for hairy mammals.

So here's the answer:

Almost certainly no more than 3 times for hairless mammals as opposed to almost certainly at least 29 times for hairy mammals.

This means, contrary to the common AAT/H claim, comparisons with other mammals do not support the idea that living a semiaquatic lifestyle likely to make you "hairless".
It's actually rare that hairlessness and aquaticism are correlated.
The AAT/H claim already falters badly just by the existence of many hairy semiaquatic mammals. It falters even more from the existence of hairy tropical semiaquatic mammals. It falters even more when you see that among aquatic and semiaquatic mammals the only hairless ones are either very large and almost certainly hairless due to the physics of thermoregulation, or are fully aquatic and have been for tens of millions of years, longer than hominids of any sort have existed. Or they are both very large and fully aquatic for tens of millions of years. When you then go further than merely counting the hairy and non-hairy species and instead look at how many times aquaticism arose in mammals, and you separate that list into hairy and not hairy, it's just devastating. No wonder none of the idea's proponents has done this obvious test.

Another AAT/H claim brought down by examining relatively easy to find facts is the idea that water makes mammals "fatties. This is because the aquatic "fatties" are all cetaceans, sirenia, and pinnipeds, all aquatic for tens of millions of years.

I've had to make some educated guesses regarding some of the hairy semiaquatic mammal groups and whether closely related mammals diverged before or after becoming semiaquatic, and whenever this was necessary I decided to to help the AAT/H case by erring on the side of assuming fewer cases of semiaquaticism evolving in mammals which remain hairy. For instance the European and North American minks are different genuses, but it seems likely enough they diverged after becoming semiaquatic. I've lumped them together as one instance of semiaquaticism arising. The muskrat and the round-tailed muskrat, despite the common names, are fairly different animals, and it seems likely enough they did evolve semiaquaticism separately, but they are the closest related among the Arvicola and in this list I assume the opposite: that they diverged after becoming semiaquatic, so they too are also lumped together as one instance of semiaquaticism arising.
There are notes after those groups of species I've done this for.

Here's the lists which show how often aquaticism and semiaquaticism arose in mammals; each instance of aquaticism or semiaquaticism arising is marked with a "+" preceding the species or group of species:

Non-hairy Aquatic or Semiaquatic Mammals

Aquaticness evolved very probably no more than 3 times, as each group branched out from a mammal which had habits etc. very much like the present-day mammals in the group and all mammals in the group are similar in habits etc.

+

Cetaceans (whales, dolphins, porpoises)

+

Sirenia (manatees, dugongs, sea cows)

+

Hippopotamidae (hippopotamus)

Hairy Aquatic or Semiaquatic Mammals

Aquaticness evolved at least 29 times -- marked "+" if the group has an evolutionary past distinct from others in the list and/or has closely related members which are not semiaquatic, esp. if a majority of the members of the group are not semiaquatic.

In those groups marked with a note there is some possibility that semiaquaticness evolved separately in the groups mentioned but for the sake of the estimate of how many times semiaquaticness arose I've assumed they did not; this makes the possible error on the side of fewer cases of semiaquaticism evolving in mammals which remain hairy.

Monotremata

+

Ornithorhynchidae (platypus)

Didelphidae

+

Didelphinae (water opossum)

Soricomorpha

Soricidae (shrews)

+

Soricinae (red-toothed shrews)

Nectogale (elegant water shrew)

Neomys (Eurasian water shrew)

Nectogalini (Old World water shrews)

Chimarrogale (Asiatic water shrews)

note: it's possible but certainly far from definite that Nectogale, Neomys,
and Nectogalini diverged from a semiaquatic ancestor; they're all in
Nectogalini but so are many non-semiaquatic shrews

Tenrecidae

+

Potamogalinae (otter shrews)

+

Limnogale (aquatic tenrec)

Talpidae

+

Talpinae (Pyrenean desman)

+

Scalopinae (star-nosed mole)

Carnivora

Canidae

Caniformia

+

Speothos (bush dog)

Mustelidae

+

Lutrinae (otters)

+

Neovison (North American minks)

+

Mustela (European mink)

note: I had previously listed
Neovision and Mustela as one instance of becoming aquatic, given what I thought was the possibility that
North American and European minks may have diverged after becoming semiaquatic.
I've learned since from Henry Pihlström (personal communication) that molecular data shows they "are not particularly closely related; they belong to independently evolved semiaquatic lineages", and that in fact based on MtDNA "nowadays many authors feel that the American minks need to be separated into a different genus" (Henry Pihlström, personal communication).

Ursidae

+

Ursus maritimus (polar bear)

Viverridae (civets, genets)

Viverrinae

+

Genetta; aka Osbornictis (aquatic genet)

Hemigalinae

+

Cynogale (otter civet)

Herpestidae (mongoose)

+

Atilax (marsh mongoose)

+

Pinnipeds (seals and walrus)

Artiodactyla

Bovidae

+

Reduncinae (lechwe)

+

Bovinae (sitatunga)

Rodentia

Hystricomorpha

+

Myocastoridae (nutria)

+

Caviidae (capybara)

+

Thryonomyidae (cane rats)

Castorimorpha

+

Castoridae (beavers)

Myomorpha

Cricetidae

Arvicolinae

+

Arvicola (water voles)

+

Ondatra (muskrat)

Neofiber (round-tailed muskrat)

note: Ondatra and Neofiber may have evolved aquaticness separately
but they are the closest related among the Arvicola; this list assumes
they diverged after becoming semiaquatic which may be incorrect

note: it's possible but certainly far from definite that Chibchanomys, Holochilus,
Nectomys, Oryzomys, and Amphinectomys all diverged from a semiaquatic ancestor;
they're all in Ichthyomyini but so are many non-semiaquatic rodents

Others not aquatic enough to be called semi-aquatic, like fishing cat; proboscis monkey; several species of mouse deer which regularly dive into water and "swim" (or rather walk) underwater, remaining there for some time (plus/minus 1 hour, albeit coming up for air during that time, breathholding for 3-4 minutes at a time) to escape predators (Greater Oriental ChevrotainTragulus napu, White-spotted Chevrotain Moschiola meminna, and Water ChevrotainHyemoschus aquaticus), they remain underwater for some time; tapirs; babyrussa (not all species of babyrussa are relatively hairless as is commonly thought); swamp rabbits and marsh rabbits (both of which are species of North American cottontail); pacas (2 species, which often try to escape predators by swimming)